Subspace frequency three was a communications channel in use in the 23rd century.
In 2266, Nyota Uhura picked up a message on subspace frequency three from the Thasian ship. (TOS: "Charlie X")
In 2267, James T. Kirk suggested Uhura try to contact GSK 783, a private transmitter on Deneva, on subspace frequency three. (TOS: "Operation -- Annihilate!")